I know how you feel. I went looking for clutch cover gaskets once, and could only find them in the kit for $15. I needed like 6 to space the cover for my cent. clutch. Wish I could help more, but that's the only good base gasket I've seen. The rest are the cheesy thin ones.

There is another option that I'm sure you've probably considered already. You can get rolls of gasket material (actually, you'd want sheets for something that thick) and cut them with a really sharp knife. The unfortunate part about this is the time involved.

got a base gasket at about 1/32inch of black material - I note that the clutch cover gasket is tan with embossed fiber pattern at about 1mm thickness - that's what I'm looking for in a base gasket - maybe I can use three black gasket to replace two tan gaskets